1. Field of the Invention
This invention relates to a distributed feedback semiconductor laser device and a method of producing the same.
2. Description of the Prior Art
Generally, the gain of a semiconductor laser device spans a relatively broad spectral range, and even when transverse modes are controlled, a perfectly single longitudinal mode can hardly be achieved. In DC operation, it may appear that a single longitudinal mode is achieved, but in practice, the oscillation is in multi longitudinal modes because of temperature variations and other factors.
A DFB (distributed feedback) semiconductor laser device is constructed with a resonator having a wavelength selectivity. In this type of semiconductor laser device, a diffraction grating is provided adjacent to an active region for laser oscillation, to selectively reflect laser light of a single wavelength within the gain spectral range, thereby attaining laser oscillation in a single longitudinal mode.
As an example of the DFB semiconductor laser devices in which stable oscillation in both single transverse and single longitudinal modes can be obtained, there has previously been proposed a semiconductor laser device as shown in FIG. 10 (refer to Appl. Phys. Lett., 34(11), pp.752-755(1979)). This type of semiconductor laser device is called an SBH-DFB laser device since it has a stripe buried heterostructure (SBH).
As shown in FIG. 10, a stripe-shaped GaAs active layer 23 formed along the resonator direction is flanked at both sides by a p-Al.sub.0.36 Ga.sub.0.64 As first buried layer 30 and is sandwiched, together with an underlying n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25, between an n-Al.sub.0.36 Ga.sub.0.64 As first cladding layer 22 and a p-Al.sub.0.36 Ga.sub.0.64 As second cladding layer 26. A diffraction grating 34 that provides a distributed feedback configuration is formed at the interface between the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 and the p-Al.sub.0.36 Ga.sub.0.64 As first buried layer 30.
In the above semiconductor laser device, laser light generated in the GaAs active layer 23 seeps into the underlying n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 through which the laser light is guided. Since the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 has a greater refractive index than that of the GaAs active layer 23, a smooth refractive index distribution with an effective refractive index increasing at portions underlying the GaAs active layer 23 is formed in lateral directions perpendicular to the resonator direction. As a result, even in high output power operations, stable oscillation of a fundamental transverse mode can be obtained.
Also, the laser light that seeped into the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 interacts with the refractive index distribution formed by the diffraction grating 34 along the resonator direction, to selectively reflect laser light of the Bragg wavelength that is determined by the pitch of the diffraction grating 34. Therefore, even when the driving current increases up to a value about three times the threshold current required to initiate oscillation, the wavelength of the oscillation does not vary and stable oscillation in a single longitudinal mode can be obtained.
However, because of its complex fabrication process sequence, the above prior art SBH-DFB laser device has had the problems of a low product yield and poor reproducibility of device characteristics. The SBH-DFB laser device shown in FIG. 10 is fabricated in the following steps, for example.
First, using an epitaxial growth technique, an n-Al.sub.0.36 Ga.sub.0.64 As first cladding layer 22, an n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25, a GaAs active layer 23, a p-Al.sub.0.36 Ga.sub.0.64 As second cladding layer 26, and a p-GaAs contact layer 27 are sequentially grown on an n-GaAs substrate 21.
Next, using a selective etching technique, the GaAs active layer 23, the p-Al.sub.0.36 Ga.sub.0.64 As second cladding layer 26, and the p-GaAs contact layer 27 are etched in a stripe pattern to form a mesa structure. Then, by a chemical etching using a photoresist mask, a diffraction grating 34 is formed over the surface of the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 exposed on both sides of the mesa structure.
Thereafter, using a liquid-phase epitaxial growth technique, a p-Al.sub.0.36 Ga.sub.0.64 As first buried layer 30 and an n-Al.sub.0.36 Ga.sub.0.64 As second buried layer 31 are grown one on top of the other on both sides of the mesa structure, to cover the surface of the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 as well as to bury the mesa structure.
Finally, a dielectric layer 37 for restricting the current injection region is deposited on a prescribed region of n-Al.sub.0.36 Ga.sub.0.64 As second buried layer 31, and after forming an n-side electrode 32 on the underside of the n-GaAs substrate 21 and a p-side electrode 33 on the upper surface of the dielectric layer 37 and the current injection region, the wafer is cleaved to form the endfaces of the resonator, thus completing the fabrication of the SBH-DFB laser device shown in FIG. 10.
As described, in the above SBH-DFB laser device, the laser light that seeped into the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 is selectively reflected by the diffraction grating 34 formed at the interface between the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 and the p-Al.sub.0.35 Ga.sub.0.64 As first buried layer 30, in order to accomplish the optical feedback. The coupling efficiency of the laser light and diffraction grating 34 is dependent on the proportion of the laser light that seeped into the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 to the total amount of laser light generated in the GaAs active layer 23.
Therefore, when the refractive index difference in lateral directions perpendicular to the resonator direction is increased in order to enhance the oscillation characteristics of single transverse modes, the seeping amount of the laser light decreases, resulting in a decrease in the coupling efficiency and thus degrading the oscillation characteristics of single longitudinal modes. Conversely, when the seeping amount of the laser light is increased in order to enhance the oscillation characteristics of single longitudinal modes, oscillation stability of the fundamental transverse mode deteriorates.
To overcome such a problem, an SBH-DFB laser device as shown in FIG. 11 has been proposed. This laser device is essentially the same as the SBH-DFB laser device shown in FIG. 10, except that the diffraction grating 34 is also formed at the interface between the GaAs active layer 23 and the underlying n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25.
However, in the above configuration, after the n-Al.sub.0.36 Ga.sub.0.64 As first cladding layer 22 and the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25 have been sequentially grown on the n-GaAs substrate 21, the growth step must be stopped temporarily in order to form the diffraction grating 34 on the surface of the n-Al.sub.0.15 Ga.sub.0.85 As optical guiding layer 25, requiring a total of three growth steps to add to the complexity of the fabrication process. Also, since the GaAs active layer 23 is grown on top of the diffraction grating 34, there is a possibility that detrimental effects may be caused to the device characteristics, such as a degradation in its crystallinity and an increase in the oscillation threshold current.
To overcome the above problems, there have been proposed DFB semiconductor laser devices that can be fabricated with only two crystal growth steps (such as disclosed in Japanese Laid-Open Patent Publication Nos. 60-145685 and 2-206191). FIG. 12 shows one of the DFB semiconductor laser devices fabricated from AlGaAs/GaAs materials. The DFB semiconductor laser device shown in FIG. 12 is fabricated with two crystal growth steps using, for example, a metal-organic chemical vapor deposition (MOCVD) method. The optical waveguide necessary for laser oscillation and the diffraction grating essential to the DFB configuration are formed during these two crystal growth steps. The fabrication method is described in detail below.
First, using an MOCVD technique, an n-GaAs buffer layer 301, an n-Al.sub.0.5 Ga.sub.0.5 As first cladding layer 302, a non-doped Al.sub.0.13 Ga.sub.0.87 As active layer 303, a p-Al.sub.0.5 Ga.sub.0.5 As carrier barrier layer 304, a p-Al.sub.0.25 Ga.sub.0.75 As optical guiding layer 307, and an n-GaAs current blocking layer 305 are sequentially grown on an n-GaAs substrate 300, as shown in FIG. 12.
Next, using photolithography and chemical etching techniques, a portion of the n-GaAs current blocking layer 305 is selectively etched to form a stripe groove (about 4 .mu.m in width) extending in parallel to the resonator direction, thereby exposing the surface of the p-Al.sub.0.25 Ga.sub.0.75 As optical guiding layer 307 in the bottom of the stripe groove. As an etchant, a mixed solution of NH.sub.4 OH, H.sub.2 O.sub.2, and H.sub.2 O is used. In this chemical etching step, by appropriately adjusting the mixing ratio of NH.sub.4 OH and H.sub.2 O.sub.2, a difference can be created in etch rate between the n-GaAs current blocking layer 305 and the p-Al.sub.0.25 Ga.sub.0.75 As optical guiding layer 307, which makes it easier to selectively etch only the n-GaAs current blocking layer 305.
Then, using the two-beam interference exposure method and a chemical etching technique, a diffraction grating 306 is formed on the surface of the optical guiding layer 307 within the stripe groove. Thereafter, using MOCVD, a p-Al.sub.0.75 Ga.sub.0.25 As second cladding layer 308 and a p-GaAs contact layer 309 are grown on top of the current blocking layer 305 and the optical guiding layer 307. Finally, after forming a p-side electrode 310 on top of the contact layer 309 and an n-side electrode 320 on the underside of the substrate 300, the wafer is cleaved into chips providing the desired resonator length, thus completing the fabrication of the DFB semiconductor laser device shown in FIG. 12.
In the above configured semiconductor laser device, since the current blocking layer 305 is formed adjacent to the active layer 303 and outside the stripe groove extending along the resonator direction, the current blocking layer 305 functions as a light absorbing layer. As a result, a difference is created in refractive index between the inside and the outside of the stripe groove, thus providing optical confinement in horizontal directions. Laser light generated in the active layer 303 seeps into the overlying optical guiding layer 307 through which the laser light is guided. The laser light that seeped into the optical guiding layer 307 interacts with the refractive index distribution formed by the diffraction grating 306 along the resonator direction, to selectively reflect laser light of the Bragg wavelength that is determined by the pitch of the diffraction grating 306. Therefore, stable oscillation of a single longitudinal mode can be obtained. Furthermore, since the current blocking layer 305 is formed on both sides of the stripe groove that provides the current confining structure, current leakage is reduced and a reduction in the threshold current is expected.
However, the DFB semiconductor laser device involves the following disadvantages.
First, by appropriately adjusting the mixing ratio of NH.sub.4 OH and H.sub.2 O.sub.2, a difference can be created in etch rate between the n-GaAs current blocking layer 305 and the p-Al.sub.0.25 Ga.sub.0.75 As optical guiding layer 307, therefore, it is possible to a certain extent to selectively etch only the n-GaAs current blocking layer 305. However, when the Al mole fraction (mixing ratio) x in the Al.sub.x Ga.sub.1-x As crystals is small, the difference in etch rate between the Al.sub.x Ga.sub.1-x As crystals and the GaAs crystals is usually not very large. Therefore, the etching endpoint in the n-GaAs current blocking layer 305 is not definite, which prevents good reproducibility of the width of the stripe groove.
Second, it is extremely difficult to apply a resist to a uniform thickness on the inside of the stripe groove in order to form the diffraction grating 306 on the surface of the optical guiding layer 307 that is exposed in the bottom of the stripe groove flanked by the thick current blocking layer 305. In reality, the photoresist remains thick on the inside of the stripe groove near the side walls of the current blocking layer 305. Also, the photoresist remaining on these portions is difficult to expose because of the interference by the side walls of the current blocking layer 305. As a result, when a conventional positive resist is used for patterning the diffraction grating 306, the formation of the diffraction grating 306 is not achieved on the portions near the side walls of the current blocking layer 305, as shown in FIG. 12.
Also, as shown in (b) of FIG. 9, the thickness of the optical guiding layer 307 becomes greater at the portions near the side walls of the current blocking layer 305 (region B in FIG. 9) where the diffraction grating 306 is not formed than at the center portion of the stripe groove (region A in FIG. 9) where the diffraction grating 306 is formed. As a result, the effective refractive index in transverse directions becomes greater in region B than in region A, so that, as shown in (a) of FIG. 9, the light intensity distribution is unevenly shifted toward region B, where the diffraction grating 306 is not formed, from region A where the diffraction grating 306 is formed. As a result, the above method has a fatal disadvantage that the fundamental transverse mode suffers very large waveguide losses since the current blocking layer 305 capable of absorbing the light of oscillation wavelength is disposed adjacent to region B.
Another problem of the semiconductor laser device of an absorption area (gain guide) structure having a stripe groove is that the far-field pattern is narrower in the lateral direction than in the vertical direction and its ellipticity (i.e., the far-field pattern in the vertical direction divided by the far-field pattern in the lateral direction) is large.